Ongoing goals of the computer industry include higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits (“IC's”). As new generations of IC products are released, their functionality increases while the number of components needed to fabricate them decreases.
Semiconductor devices are constructed from a silicon or gallium arsenide wafer through a process that comprises a number of deposition, masking, diffusion, etching, and implanting steps. Usually, many individual devices are constructed on the same wafer. When the devices are separated into individual rectangular units, each takes the form of an IC die. In order to interface a die with other circuitry, it is common to mount it on a leadframe or on a multi-chip module substrate that is surrounded by a number of lead fingers. Each die has bonding pads that are then individually connected in a wire-bonding operation to the leadframe's lead finger pads using extremely fine gold or aluminum wires. The assemblies are then packaged by individually encapsulating them in molded plastic or ceramic bodies.
IC packaging technology has shown an increase in semiconductor chip density (the number of chips mounted on a single circuit board or substrate) that parallels the reduction in the number of components that are needed for a circuit. This results in packaging designs that are more compact, in form factors (the physical size and shape of a device) that are more compact, and in a significant increase in overall IC density. However, IC density continues to be limited by the space (or “real estate”) available for mounting individual die on a substrate.
To further condense the packaging of individual devices, packages have been developed in which more than one device can be packaged at one time at each package site on a leadframe strip. Each package site on a leadframe strip is a structure that provides mechanical support for the individual IC devices. It also provides one or more layers of interconnect lines that enable the devices to be connected electrically to surrounding circuitry. Of importance to complicated packaging designs are considerations of input/output count, heat dissipation, matching of thermal expansion between a motherboard and its attached components, cost of manufacturing, ease of integration into an automated manufacturing facility, package reliability, and easy adaptability of the package to additional packaging interfaces such as a printed circuit board (“PCB”).
Various chip-on-board (“COB”) techniques are used to attach different semiconductor die to a PCB. COB techniques include flip chip attachment, wire bonding, and tape automated bonding (“TAB”).
Flip chip attachment consists of attaching a flip chip to a PCB or to another substrate. A flip chip is a semiconductor chip that has a pattern or array of terminals spaced around on an attachment surface on the chip for face-down mounting to a substrate. Generally, the attachment surface of the flip chip has one of the following electrical connectors: ball grid array (“BGA”) or slightly larger than IC carrier (“SLICC”). BGA is an electrical connector configuration having an array of minute solder balls disposed on the attachment surface of the flip chip for attaching to the substrate. SLICC is similar to the BGA, but has a smaller solder ball pitch and diameter than the BGA.
With the BGA or SLICC, the solder or other conductive ball arrangement on the flip chip must be a mirror image of the connecting bond pads on the PCB so that precise connection can be made. The flip chip is bonded to the PCB by melting (refluxing) the solder balls. The solder balls may also be replaced with a conductive polymer or gold stud bumps bonded using a conductive polymer.
Wire bonding attachment and TAB attachment generally begin with attaching a semiconductor chip to the surface of a small PCB with an appropriate adhesive such as an epoxy. With wire bonding attachment, wires are then attached, one at a time, to each bond pad on the semiconductor chip and extend to a corresponding metal lead or trace end on the PCB. With TAB, the ends of metal leads that are carried on an insulating tape are respectively attached to the bond pads on the semiconductor chip and to the lead or trace ends on the PCB. An encapsulant is then generally used to cover the bond wires and metal tape leads to prevent damage or contamination.
In some cases, multi-chip devices can be fabricated faster and more cheaply than a corresponding single IC chip that incorporates all the same functions. Current multi-chip modules typically consist of a PCB substrate onto which a set of separate IC chip components is directly attached. Such multi-chip modules have been found to increase circuit density and miniaturization, improve signal propagation speed, reduce overall device size and weight, improve performance, and lower costs—all primary goals of the computer industry.
However, such multi-chip modules can be bulky. IC package density is determined by the area required to mount a die or module on a circuit board. One method for reducing the board size of multi-chip modules and thereby increase their effective density is to stack the die or chips vertically within the module or package.
In one design, a pair of IC die is mounted on opposite sides of a leadframe paddle. Gold or aluminum wires then connect the wire bonding pads on both the upper die and the lower die with the ends of their associated leadframe lead extensions.
Other representative designs for mounting multiple semiconductor IC chips in a single, multi-chip package include: two chips mounted on two leadframe paddles, one chip mounted over a paddle and one below mounted on a board, and one chip attached on top of a larger chip that is attached below to a paddle. These and other configurations have also been extended to include three or more chips mounted together vertically in a single package.
Such designs are improvements over prior multi-chip package and system-in-a-package (“SiP”) designs that combined several semiconductor die and associated passive components (“passives”) side by side in a single, horizontal layer. Combining them into a single horizontal layer used board space inefficiently by consuming large substrate areas, and afforded less advantage in circuit miniaturization.
However, multi-chip modules, whether vertically or horizontally arranged, can also present problems because they usually must be assembled before the component chips and chip connections can be tested. That is, because the electrical bond pads on a die are so small, it is difficult to test die before assembly onto a substrate. Thus, when die are mounted and connected individually, the die and connections can be tested individually, and only known-good-die (“KGD”) that are free of defects are then assembled into larger circuits. A fabrication process that uses KGD is therefore more reliable and less prone to assembly defects introduced due to bad die. With conventional multi-chip modules, however, the die cannot be individually identified as KGD before final assembly, leading to KGD inefficiencies and assembly process yield problems.
Two of the common die stacking methods are: (a) larger lower die combined with a smaller upper die, and (b) so-called same-size die stacking. With the former, the die can be very close vertically since the electrical bond pads on the perimeter of the lower die extend beyond the edges of the smaller die on top. With same-size die stacking, the upper and lower die are spaced more vertically apart to provide sufficient clearance for the wire bonds to the lower die. As discussed, both these methods have inherent KGD and assembly process yield loss disadvantages since KGD cannot be used for fabricating these configurations.
Another previous design is package level stacking. This concept includes stacking of two or more packages. KGD and assembly process yields are not an issue since each package can be tested prior to assembly, allowing KGD to be used in assembling the stack. But package level stacking can pose other problems. One problem is package-to-package assembly process difficulties caused by irregularities in the flatness/coplanarity of the lower package. Another problem results from the increased stiffness of the overall assembly, which can lead to reduced board level reliability. Still another problem can arise from poor heat dissipation from the upper package.
Thus, despite the advantages of recent developments in semiconductor fabrication and packaging techniques, there is a continuing need for improved packaging methods, systems, and designs for increasing semiconductor die density in PCB assemblies.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.